Depends entirely on your flock, your surroundings/environment, and your own personal comfort level.
I live in the suburbs of a major city, on a small lot in a neighborhood. When I got my first two chickens, I had a mobile coop/pen that they stayed in all of the time, unless I was home and outside working in the (fenced) yard myself. After the first year, I became a little more lax, and let the chickens out whenever I was home, whether or not I was outside. Now, I let my chickens out into my yard all day, every day.
Yes, there are risks--there's a hawk whose territory covers my entire cul-de-sac, and a couple of stray cats, and raccoons--but I feel the benefits of them getting free range of my back yard far outweighs those risks, and also the cons posed by keeping them penned up like I used to. All I can do is take all the appropriate precautions I can.